A light emitting diode (LED) is a semiconductor device constituted mainly by group III-V compound semiconductor materials, for instance. Such semiconductor materials have a characteristic of converting electricity into light. Hence, when a current is applied to the semiconductor materials, electrons therein would be combined with holes and release excessive energy in a form of light, thereby achieving an effect of luminosity.
When the luminous wavelength of the LED gradually shifts from blue to the ultraviolet wavelengths of light, due to the lattice constant, the thermal expansion coefficient and the chemical properties of the sapphire are different from those of the gallium nitride/aluminum nitride, the gallium nitride/aluminum nitride growing on the heterogeneous substrate (such as, silicon substrate, silicon carbide substrate and sapphire substrate) possesses a large quantity of defects and dislocations. As the growth thickness of a buffer layer (e.g., gallium nitride or aluminum nitride) increases, these dislocations may extend toward a thickness direction of the buffer layer. As such, the lattice dislocation reduces the light emitting efficiency of the LED and shortens lifetime thereof.
Various approaches have been proposed to reduce such defects and dislocations. Although dislocation extending upwards may not be apt to exist in a portion of the buffer layer with certain approaches, the defect density of the LED remains relatively high as the luminous wavelength of the LED shifts to the ultraviolet wavelengths of light, the buffer layer requires to grow to a certain thickness (such as, greater than 10 μm) and coalescence of the nitride semiconductor layer is difficult to be achieved, thereby drastically decreasing the luminous intensity of the LED at the ultraviolet wavelengths. As a result, manufacturers in the pertinent art endeavour to develop LED with satisfactory luminous efficiency.